Consider the centre of mass of the system to be the rotational axis. The moment of inertia is simply the sum of the individual moments. Each moment is given as mr^2. What does that make the moment of inertia?

The answers look fine to me.
Be sure to convince yourself that the planets are rotating in the same direction (both clockwise or both counterclockwise), so you can add the angular momenta of the two instead of subtracting them.